Leak inspection of cans immediately after retort sterilization following filling and sealing is conducted with an inline tap tone inspection device disposed in the production line. Such leak inspection is conducted by inspecting the internal pressure of the can corresponding to an actual leak amount, rather than by directly measuring the actual leak amount of the can. Further, the internal pressure inspection of the can is conducted by measuring the peak frequency (resonance frequency) of the tap tone generated correspondingly to the internal pressure of the can, rather than by directly measuring the pressure inside the can. Thus, the leak inspection of the can is conducted by inspecting the tap tone of the can. However, when the defects are extremely small, such as pinholes, the amount of inflowing air is very small. Therefore, the change in the internal pressure is small immediately after the production. As a result, cans with such defects are not detected in the tap tone inspection and can be shipped as acceptable products. Accordingly, a method is used in which the tap tone inspection of the can is conducted after the cans are stored for several days before shipping. With such a method, because time is sufficiently secured for the external air to flow into the can even if the defects are pinholes, the internal pressure of the can rises sufficiently and this rise can be detected by tap tone inspection. It is noted that the cans are stored after they have been packed into carton cases. A case tap inspection device having heads whose arrangement matches an arrangement of cans is used to conduct tap inspection of the cans in this state. The case tap inspection device is suitable as a means for detecting the pinhole defects, but since it inspects the cans inside a carton case, the tap tone with a peak frequency corresponding to the internal pressure of the can possibly happen to be not generated because of contact with adjacent other cans. Further, a tap tone with a peak frequency corresponding to the internal pressure similarly cannot be obtained and acceptability of the internal pressure of the can cannot be judged when the can bottom (corresponds to a steel lid in a three-piece can) is strongly pressed against the internal surface of the bottom portion of the carton case. Thus, the problem associated with the leak inspection with a tap tone inspection device is that accurate inspection results cannot be obtained for certain states of cans inside the carton case.
Furthermore, when the contents contain a solid material or a high viscosity material, such contents adhere to the bottom of the can and therefore a tap tone with a peak frequency corresponding to the internal pressure of the can cannot be obtained. Depending on the amount of the adhesion, the tap tone itself is decreased and cannot be judged. In other words, the problem associated with the tap tone inspection method is that the acceptability of the internal pressure of the can cannot be accurately judged.
In order to inspect the cans that are placed inside a carton case, the case tap tone inspection device has tap tone inspection heads corresponding to the rows of cans and is capable of inspecting cans in a plurality of rows at the same time. The inspection timing is determined by detecting with a photoelectric switch the end of the carton case moving on a conveyor. The arrival of the can directly below the tap tone inspection head may be determined by counting output pulses of the encoder mounted on a roller shaft of the conveyor. At this time, a pulsed current flows in the coil portion of the tap tone inspection head, the can bottom is sucked in and released, and a tap tone is generated. The tap tone is collected by a microphone located inside the tap tone inspection head, amplified, filtered, and then converted into a digital signal with an A/D converter. The digital signal (waveform data) is processed to a fast Fourier transformation (FFT) by a microcomputer and a peak frequency of the tap tone is found. Since this peak frequency changes correspondingly to the internal pressure of the can, whether the internal pressure of the can is acceptable is judged by upper limit and lower limit frequency thresholds. Generally, in the cans with a negative pressure, the peak frequency of tap tone increases as the internal pressure of the can decreases. Therefore, if a lower limit threshold is set as a target with respect to the peak frequency corresponding to the internal pressure of can and a peak frequency of a can is inspected on the basis of the lower limit threshold so that cans with lower peak frequency than this threshold are rejected, it is possibly to reject the defective cans with increased internal pressure, that is, an opened orifice.
When a pinhole is opened in a can, bacteria or the like may come into the can through the pinhole within several days after the can has been manufactured and the contents of the can may rot. Rotten contents generate gas that is released inside the can, and the gas is accumulated inside the can when the pinhole opened in the can is very small or the pinhole is blocked by the rotten contents. When the release of gas is small, the internal pressure of the can rises. Therefore, this increase in internal pressure can be detected by the lower limit threshold of the peak frequency of tap tone.
However, when the can is under positive pressure, the peak frequency of the tap tone increases as the internal pressure of the can increases, by contrast with the case of negative pressure. Therefore, even if the pressure is initially negative, if rotting process advances and a large amount of gas is released inside the can, the internal pressure of the can changes from negative to positive. As a result, the peak frequency of the tap tone increases as the internal pressure of the can rises. Thus, in such a state that the internal pressure of the can is under a positive pressure, although the can is defective and a hole is opened therein, the peak frequency of the tap tone enters the acceptable range (equal to or above the lower limit threshold) due to increasing of the internal pressure of the can. As a result, the can is determined to be acceptable although the can is defective and has a hole. In other words, the case tap tone detector cannot effectively function for cans in which a pinhole is blocked by rotten contents, and such defective cans can be missed.
In order to prevent such a problem, the case tap tone detector is provided with a displacement sensor that measures the height of the can bottom, and the distance to the bottom (or lid) of the can that has arrived to a position immediately below the detector is measured. This approach is based on the assumption that in a can in which the internal pressure of the can has turned into a positive pressure due to the effect of gas produced by rotten contents, the can lid or bottom becomes convex due to the expansion. Therefore, when the distance to the can lid or can bottom is measured with the displacement sensor and the measured value is less than a fixed value (this indicates the convex state), the can is rejected as an expanded can even if the peak frequency of tap tone is normal.
As described above, the peak frequency of tap tone may change due to the effect of pressure applied to a can from the adjacent can during inspection. Further, as a result of stacking during storage, a can lid or can bottom may be strongly pressed against the inner surface of the bottom portion of the carton case, and when a recess appears in the carton case, the volume of sound emitted from the can may be insufficient during the tap tone inspection. When the recess appears in the carton case, the relative positions of the can and the displacement sensor shift from those in the normal case. As a result, the function of detecting an expanded can may not operate properly. In order to prevent the above-described inconvenience, an invention designed to reduce the effect produced by carton case on the tap tone is known (see, for example, Patent Document 1). This invention is based on the idea of blowing the air into a carton case having cans inside thereof and separating the carton case from the can lid or can bottom. This method effectively can reduces the effect produced by the carton case on the tap tone, but has demerits such as a high cost and decreased inspection speed since time is required to blow the air.
Further, when a can is filled with contents containing solids, such as corn potage soup, or contents of high viscosity, such as oshiruko (red-bean soup), these contents adhere to the can bottom, the effective weight of the panel portion is increased, and a frequency spectrum of the tap tone corresponding to the internal pressure of the can possibly cannot be obtained. Different from the effect produced by concavities of the carton case, this effect is difficult to reduce by blowing air.
An internal pressure inspection method for a can by which an amount of displacement of a central portion of the can lid from a predetermined reference position is measured with a (distance) sensor and whether the internal pressure of the can is acceptable is judged on the basis of this displacement amount has been known as an internal pressure inspection method for a can that does not rely on tap tone inspection (see, for example, the below-described Patent Documents 2 to 4); with this method the attention is focused on a concave-convex deformation of the can lid corresponding to the internal pressure of the can. According to the inventions disclosed in these patent documents, the central portion of the can lid is used as a displacement inspection point, and a seamed upper end portion (two-point average value) on both sides of the can lid that passes through the central portion and is parallel to the conveying direction, a seamed upper end portion (two-point average value) on both sides of the can lid that crosses the conveying direction, or an inner side (two-point average value) on the seamed upper end portion of the can lid is used as the reference position for displacement. In Patent Document 2, the seamed upper end portion on both sides of the can lid that crosses the conveying direction is used as a reference position for displacement in order to prevent a phenomenon according to which a displacement sensor pikes up not only a measurement signal of the seamed upper end portion that is an object, but also a measurement signal of the seamed upper end portion of the adjacent can that is not an object and the measurement accuracy at the seamed upper end portion that is an object decreases accordingly. Further, in Patent Document 3, the seamed upper end portion on both sides of the can lid that pass through the central portion of the can lid and is parallel to the conveying direction is set as a reference position for displacement and the amount of displacement of the central portion of the can lid from this reference position is compared with a preset threshold, whereby determining whether the internal pressure of the can is acceptable. Further, in Patent Document 4, a predetermined correction amount is added to the displacement amount of the leading can and the last can that is measured with the displacement sensor, whereby preventing an inconvenient event in which the measured displacement amounts of the leading can and the last can are undervalued lower than actual and the acceptable products are erroneously judged as defective products and rejected. Further, in Patent Documents 2 to 4, the cans are conveyed by a conveyor in a carton case. Therefore, the measurement signal transmitted by the displacement sensor onto the measurement object has to be capable of penetrating through the carton case. For this reason, an eddy current displacement sensor is used as the displacement sensor.
However, the measurement principle of the eddy current displacement sensor involves the operations of acting upon the measurement object with a high-frequency magnetic flux (magnetic field), inducing an eddy current in the surface of the measurement object (conductor), and using a mutual induction effect according to which the impedance (approximately equal to the induced reactance) of the displacement sensor (coil) itself is changed by the magnetic flux generated by the induced eddy current. Thus, the variation amount of the impedance strongly depends on the distance from the displacement sensor to the measurement object. Therefore, by picking up the displacement amount of the impedance as an electric signal, it is possible to find the distance from the sensor to the measurement object. Accordingly, the measurement object has to be positioned in a range of electromagnetic interference with the sensor and the (effective) area of the measurement object has to be sufficiently large for the magnetic flux to pass (permeate) therethrough. Therefore, a magnetic flux generated by the sensor cannot to pass in a sufficient amount through the measurement object region with a small thickness with respect to the radial direction, such as the seamed upper end portion of the can lid, that is, a region with a small cross-sectional area in the radial direction. As a result, it is thought that an accurate distance from the sensor to this region is difficult to obtain. Further, as described Patent Document 2, when three eddy current displacement sensors are disposed side by side in a row in the radial direction above the can lid and in this arrangement the central sensor measures a distance to the central portion of the lid can and the sensors on both sides measure the distance to the seamed upper end portions of the can lid at the same time, magnetic fluxes of the sensors interfere with each other. As a result, an accurate distance from each sensor to each measurement object region is similarly thought to be difficult to measure.    Patent Document 1: Japanese Patent Application Laid-open No. 2006-38826.    Patent Document 2: Japanese Patent Application Laid-open No. H8-219915.    Patent Document 3: Japanese Examined Patent Application No. H5-38891.    Patent Document 4: Japanese Patent Application Laid-open No. S63-302337.